The commercial freezing of food products, especially soft food products such as shrimp, scallops, vegetables, meat and the like is often carried out rapidly to minimize water loss and damage to the product. Quick freezing may be carried out using cryogenic substances such as liquid nitrogen and solid carbon dioxide.
Among the more common cryogenic quick freezing techniques are those employing either a freezing tunnel or an immersion bath. Freezing tunnels are elongated chambers which may contain spaced apart nozzles that are used to spray the cryogenic substance on top of the food product as it passes through the tunnel on a conveyor. Heat exchange between the food product and the cryogenic substance produces cryogenic vapor which is circulated by fans to improve cooling efficiency.
The liquid cryogen immersion bath system provides for a vessel containing a liquid cryogen (e.g. liquid nitrogen) and a conveyor passing there through to thereby immerse the food product in the liquid cryogen. The immersion bath system is advantageous over tunnel systems because freezing is faster. However, the efficiency of the immersion bath system is generally no better than most tunnel systems, typically because the cryogenic vapor generated in the immersion bath is not effectively utilized.
In an effort to improve the efficiency of quick freezing, systems have been developed that utilize the combination of a liquid immersion bath and a freezer tunnel. In such systems the unfrozen product is introduced into the liquid immersion bath and moved along the bath on a conveyor. The immersion bath provides the first stage of cooling. A cryogenic vapor is produced via heat exchange between the food product and the liquid cryogen. The food product which emerges from the bath is passed into a freezing tunnel which is typically positioned downstream of the immersion bath. The cryogenic vapor is drawn into the tunnel by fans to complete the freezing process. Such systems are disclosed, for example, in R.C. Webster et al., U.S. Pat. No. 3,485,055; I. Rasovich, U.S. Pat. No. 4,403,479; L. Tyree, Jr. et al., U.S. Pat. No. 4,783,972; P. Gibson, U.S. Pat. No. 4,843,840; A. Acharya et al., U.S. Pat. No. 4,852,358; I. Rasovich, U.S. Pat. No. 4,858,445; E. Kiczek et al., U.S. Pat. No. 5,168,723; E. Kiczek et al., U.S. Pat. No. 5,220,802; E. Kiczek et al., U.S. Pat. No. 5,220,803; and R. Howells, U.S. Pat. No. 5,267,490.
The combination of a liquid immersion bath and a downstream freezer tunnel, particularly those using circulated cryogenic vapor provide improved cooling efficiency over systems using the immersion bath alone or the freezer tunnel alone. However, the combined system suffers from a number of disadvantages. Since the freezer tunnel extends horizontally from the immersion bath, the combined system occupies a large amount of floor space and is relatively expensive to construct. Furthermore, high energy fans are needed to draw the cryogenic vapor from the immersion bath into and through the elongated freezer tunnel, thereby increasing the cost of operating the combined system.
In addition, the combined systems of the prior art typically have an entrance leading into the immersion bath and an exhaust outlet at the remote end of the freezer tunnel. In this type of arrangement it is difficult to control the flow of cryogenic vapor within the freezing apparatus and also difficult to prevent loss of the same.
It would therefore be a significant advance in the art of freezing food products if a combined freezer system, utilizing both an immersion bath and a freezer tunnel could be made more compact, less costly and operate more efficiently than present combined freezer systems.